In the field of mobile communication, a variety of information such as image and data in addition to speech becomes transmission targets in recent years. Accompanying this, the demand for reliable and high-speed transmission has increased. However, when high-speed transmission is carried out in mobile communications, the influence of delayed waves due to multipath cannot be ignored, and transmission performance deteriorates due to frequency selective fading.
As one of countermeasure techniques for frequency selective fading, multicarrier communication represented by OFDM (Orthogonal Frequency Division Multiplexing) communication becomes a focus of attention. In multicarrier communication, data is transmitted using a plurality of subcarriers where transmission speed is suppressed to an extent not to cause frequency selective fading. Particularly, in OFDM communication, the frequencies of a plurality of subcarriers where data is arranged are orthogonal to each other, so that it is possible to achieve the maximum frequency efficiency in multicarrier communication schemes and realize multicarrier communication in a relatively simple hardware configuration. Consequently, OFDM communication is attracted attention as a communication method to be employed in cellular scheme mobile communications, and is studied variously. In addition, in OFDM communication, to avoid ISI (Intersymbol Interference) the tail part of an OFDM symbol is attached to the beginning of that OFDM symbol as a cyclic prefix (CP). By this means, for the receiving side, it is possible to avoid ISI as long as the delay time of delay waves stays within the time length of CP (hereinafter “CP length”).
Further, studies have been recently conducted for multicast communication. Multicast communication is point-to-multipoint communication, unlike unicast communication, which is point-to-point communication. That is, in multicast communication, one radio communication base station apparatus (hereinafter abbreviated as “base station”) transmits the same data (i.e. multicast data) to a plurality of radio communication mobile stations (hereinafter abbreviated as “mobile station”) at the same time. By this multicast communication, in mobile communication systems, for example, delivery services of music data and video image data and broadcast services such as television broadcast are realized.
Further, in multicast communication, one base station transmits the same multicast data to a plurality of mobile stations at the same time as described above, and, when one cell is divided into a plurality of sectors, multicast data is the same in a plurality of sectors. Furthermore, when one cell is divided into a plurality of sectors, the same multicast data is transmitted for a plurality of sectors at the same time, and therefore, a mobile station near the cell boundary receives multicast data for a plurality of sectors in a mixed manner.
Here, in cases of using the OFDM scheme in multicast communication, when a mobile station near the sector boundary receives a plurality of identical OFDM symbols, which are transmitted to a plurality of sectors at the same time, within the CP length, the mobile station receives them in a state where these OFDM are combined and received power is amplified. This method of transmitting the same data using the same resources (i.e. the same time and the same frequency) via a plurality of paths is referred to as “SFN (Single Frequency Network) transmission.” In SFN transmission, a mobile station is able to receive data without inter-sector interference, so that it is possible to carry out high quality transmission with a lower error rate.
Further, to compensate channel fluctuation (phase fluctuation and amplitude fluctuation) of this combined signal by channel estimation, a channel estimation value of the combined signal is needed. That is, in multicast communication using the OFDM scheme, pilots used to find the channel estimation value need to be transmitted to a plurality of sectors at the same time, as in the case of multicast data. That is, the multicast pilot needs to be a common pilot between a plurality of sectors.
On the other hand, in unicast communication, in cases of dividing one cell into a plurality of sectors, different data (i.e. unicast data) is transmitted to a plurality of sectors. That is, unicast data is unique per sector. Consequently, in unicast communication, as for pilots used to find the channel estimation values, different pilots for unicast data (i.e. unicast pilots) need to be transmitted to a plurality of sectors similar to unicast data. That is, unicast pilots needs to be individual between a plurality of sectors.
Multicast communication adopts a scheme of transmitting information only to specific mobile stations subscribing to services including news groups, and, meanwhile, broadcast communication adopts a scheme of transmitting information to all mobile stations like current TV broadcasting or radio broadcasting. However, multicast communication and broadcast communication are similar in involving point-to-multipoint communication, and, a description using MBMS (Multimedia Broadcast/Multicast Service) which combines multicast communication and broadcast communication, may be given depending on documents. Further, a description may be given using broadcast communication instead of multicast communication, depending on documents.
Here, when one cell is divided into a plurality of sectors, to reduce interference between sectors, an orthogonal pilot channel sequence, which is orthogonal between sectors, is set as a unicast pilot sequence. For example, when one cell is formed with three sectors, that is, sectors 1 to 3, as shown in FIG. 1, a sequence formed with all “1's” (the amount of phase rotation θ=0) is set for sector 1, a sequence by multiplying the sequence of sector 1 and a sequence formed with 1, exp(j2p/3), and exp(j4p/3) . . . (the amount of phase rotation θ=2p/3) is set for sector 2, and, a sequence by multiplying the sequence of sector 1 and a sequence formed with 1, exp(j4p/3), and exp(j2p/3) . . . (the amount of phase rotation θ=4p/3) is set for sector 3. That is, as shown in FIG. 1, unicast pilot sequences of sector 1 to 3, are orthogonal to each other based on one unit (orthogonal sequence unit) formed with three chips in the combination of 1, exp(j2p/3), and exp(j4p/3). Further, each unicast pilot sequence is formed with a plurality of identical orthogonal sequence units. For example, the unicast pilot sequence for sector 1 is a repetition of the orthogonal sequence unit “1, 1, 1,” the unicast pilot sequence for sector 2 is a repetition of the orthogonal sequence unit “1, exp(j2p/3), exp(j4p/3),” and the unicast pilot sequence for sector 3 is a repetition of the orthogonal sequence unit “1, exp(j4p/3), exp(j2p/3).”
Then, this unicast pilot arrangement method is studied as shown in FIG. 1 (see Non-patent Document 1). In FIG. 1, for ease of description, a case is shown as one example where one OFDM symbol is formed with subcarriers f1 to f25 and where one sub-frame is formed with OFDM symbols #1 to #7. The same applies to the drawings below. In the example shown in FIG. 1, the unicast pilot is arranged to subcarriers f1, f7, f13, f19 and f25 in OFDM symbol #1 and subcarriers f4, f10, f16 and f22 in OFDM symbol #5 in the sectors. That is, for example, the unicast pilot “1” of sector 1, the unicast pilot “1” of sector 2 and the unicast pilot “1” of sector 3 are multiplexed on subcarrier f1 in OFDM symbol #1 on the channel. The unicast pilot “1” of sector 1, the unicast pilot “exp(j2p/3)” of sector 2 and the unicast pilot “exp(j4p/3)” of sector 3 are multiplexed on subcarrier f4 in OFDM symbol #5 on the channel. The unicast pilot “1” of sector 1, the unicast pilot “exp(j4p/3)” of sector 2 and the unicast pilot “exp(j2p/3)” of sector 3 are multiplexed on subcarrier f7 in OFDM symbol #1 on the channel. That is, unicast pilots of a plurality of sectors are multiplexed on the same frequency of the same time. By adopting this arrangement, it is possible to arrange unicast pilots in one sub-frame both in the frequency domain and the time domain all over.
On the other hand, by contrast with this unicast pilot arrangement method, a multicast pilot arrangement method is studied as shown in FIG. 2 (see Non-patent Document 2). As shown in the above description, multicast pilots of a plurality of sectors are required to be arranged to the same frequency of the same time. In the example shown in FIG. 2, in each sector, the multicast pilot is arranged to subcarriers f4, f10, f16 and f22 in OFDM symbol #1 and subcarriers f1, f7, f13, f19 and f25 in OFDM symbol #5. By adopting this arrangement, similar to a unicast pilot, it is possible to arrange multicast pilots in one sub-frame both in the frequency domain and in the time domain all over. Further, conventionally, to sufficiently acquire accuracy of channel estimation for multicast data in all frequencies, as shown in FIG. 2, the number of multicast pilots and the number of unicast pilots arranged in one sub-frame are the same.    Non-patent Document 1: 3GPP TSG RAN WG1 Meeting #46, R1-062099, Tallinn, Estonia, Aug. 28-Sep. 1, 2006, NTT DoCoMo, Ericsson, Fujitsu, Intel Corporation, KDDI, Mitsubishi Electric, NEC, Panasonic, Qualcomm, Sharp, and Toshiba Corporation, “Orthogonal Reference Signal Structure for Sectored Beams in E-UTRA Downlink”    Non-patent Document 2: 3GPP TSG RAN WG1 Meeting #44bis, R1-060779, Athens, Greece, 27-31 Mar. 2006, NTTDoCoMo, Mitsubishi Electric, NEC, Sharp, Toshiba Corporation, “Investigations on Pilot Channel Structure for MBMS in E-UTRA Downlink”